下面是Rails是如何做到的:

   def underscore(camel_cased_word)
     camel_cased_word.to_s.gsub(/::/, '/').
       gsub(/([A-Z]+)([A-Z][a-z])/,'\1_\2').
       gsub(/([a-z\d])([A-Z])/,'\1_\2').
       tr("-", "_").
       downcase
   end

看看Ruby Facets的蛇案

处理以下情况，如下所示:

"SnakeCase".snakecase         #=> "snake_case"
"Snake-Case".snakecase        #=> "snake_case"
"Snake Case".snakecase        #=> "snake_case"
"Snake  -  Case".snakecase    #=> "snake_case"

来自:https://github.com/rubyworks/facets/blob/master/lib/core/facets/string/snakecase.rb

class String

  # Underscore a string such that camelcase, dashes and spaces are
  # replaced by underscores. This is the reverse of {#camelcase},
  # albeit not an exact inverse.
  #
  #   "SnakeCase".snakecase         #=> "snake_case"
  #   "Snake-Case".snakecase        #=> "snake_case"
  #   "Snake Case".snakecase        #=> "snake_case"
  #   "Snake  -  Case".snakecase    #=> "snake_case"
  #
  # Note, this method no longer converts `::` to `/`, in that case
  # use the {#pathize} method instead.

  def snakecase
    #gsub(/::/, '/').
    gsub(/([A-Z]+)([A-Z][a-z])/,'\1_\2').
    gsub(/([a-z\d])([A-Z])/,'\1_\2').
    tr('-', '_').
    gsub(/\s/, '_').
    gsub(/__+/, '_').
    downcase
  end

  #
  alias_method :underscore, :snakecase

  # TODO: Add *separators to #snakecase, like camelcase.

end

Rails的ActiveSupport这样 使用以下方法向字符串中添加下划线:

class String
  def underscore
    self.gsub(/::/, '/').
    gsub(/([A-Z]+)([A-Z][a-z])/,'\1_\2').
    gsub(/([a-z\d])([A-Z])/,'\1_\2').
    tr("-", "_").
    downcase
  end
end

然后你可以做一些有趣的事情:

"CamelCase".underscore
=> "camel_case"

下面是Rails是如何做到的:

   def underscore(camel_cased_word)
     camel_cased_word.to_s.gsub(/::/, '/').
       gsub(/([A-Z]+)([A-Z][a-z])/,'\1_\2').
       gsub(/([a-z\d])([A-Z])/,'\1_\2').
       tr("-", "_").
       downcase
   end

一行Ruby实现:

class String
   # ruby mutation methods have the expectation to return self if a mutation occurred, nil otherwise. (see http://www.ruby-doc.org/core-1.9.3/String.html#method-i-gsub-21)
   def to_underscore!
     gsub!(/(.)([A-Z])/,'\1_\2')
     downcase!
   end

   def to_underscore
     dup.tap { |s| s.to_underscore! }
   end
end

所以“SomeCamelCase”。To_underscore # =>"some_camel_case"

有一个Rails内置的方法叫做“下划线”，你可以使用它来实现这个目的

"CamelCaseString".underscore #=> "camel_case_string" 

“下划线”方法通常可以被认为是“camelize”的逆方法。